Electrically programmable semiconductor fuses, or electrical fuses in short as it is referred to in this invention, have been used in semiconductor circuits to provide alterations in the functionality of the circuitry. Typical examples of applications of electrical fuses include: providing redundancy to enable repairs of imperfect chips, storage of secure and permanent information, selection of a particular configuration for chip operation, tuning analogue circuit components, optimizing overall circuit performance, and/or replacing defective circuit elements with redundant circuit elements.
Electrical fuses are programmed by the physical alteration of the structure of the electrical fuses. The most commonly used structure of electrical fuses employs a vertical stack comprising a semiconducting material and a conducting material. While the most common material for the vertical stack is polysilicon and silicide, other semiconducting materials and other conducting materials may be utilized if similar electromigration properties can be found in the combined stack of the two materials. This stack is patterned such that a narrow and long piece of material, called “fuselink” or “fuse neck,” is adjoined by two large plates, called “cathode” and “anode” respectively, depending on the polarity of electrical bias applied to the electrical fuse during the programming. Electrical current of relatively high density flows through the fuselink when a sufficiently high voltage bias is applied across the cathode and the anode. The programming current may be high enough to cause the electrical fuses to rupture by a sudden increase in temperature in the physical structure of the electrical fuses. This type of programming is commonly referred to as “rupture mode programming.” Alternatively, the level of the programming current may be moderated to cause a controlled electromigration of the material inside the electrical fuse structure. This alternative mode of programming is commonly referred to as “electromigration mode programming.” Both types of programming method raise the resistance of the programmed fuse compared to that of intact fuses.
By measuring the resistance of electrical fuses, it can be determined whether the electrical fuse has been programmed or intact. While it may not be necessary to measure the exact value of the fuse resistance to determine the state of the fuse, it is generally necessary to determine whether the fuse resistance has been raised by a significant amount above the detection limit of the sensing circuitry. Typically, this is done by setting the resistance for a reference resistor at a value about 3˜10 times that of an intact electrical fuse and comparing the resistance of the fuse with that of the reference resistor. A difference between the resistance of the reference resistor and the resistance of an intact fuse is often necessary to insure margin in the functionality of the sensing circuitry under adverse operating conditions of the chip.
Rupture mode programming in general tends to produce a distribution of programmed electrical fuse resistance with a significant fraction of low resistance values. Considering that millions of electrical fuses are often used in an array of electrical fuse memory, a failure rate even at a parts per million (ppm) level could be a reliability issue. Electromigration mode programming tends to generate less of the low resistance tails, and hence, better reliability in general. An example of the performance of P-doped electrical fuses in the electromigration mode can be found in Kothandaraman et al., “Electrically Programmable Fuse (eFUSE) Using Electromigration in Silicides,” IEEE Electron Dev. Lett. Vol. 23, No. 9, September 2002, pp. 523-525. Kothandaraman et al. describes an electrically programmable P-doped fuse with P-doped polysilicon in the cathode, fuselink, and anode.
Despite the general improvement in the distribution of post-programming resistance of electrical fuses through the use of electromigration mode programming, not all fuses produce a post-programming resistance distribution with high resistance values even in an electromigration mode. The distribution of the resistance of programmed fuses is also dependent on the design of fuses as well; some producing more low resistance values for programmed fuses, while some others produce less low resistance values. Of critical importance among the features of the design is the doping of each component of the electrical fuse, i.e., the cathode, the fuselink, and the anode.
Since the sense circuitry interprets any fuse with resistance less than that of the resistance of the reference resistor as an intact, any programmed fuse with its resistance lower than that of the reference resistor is erroneously sensed as an intact fuse during a sense operation. An improved electrical fuse structure that produces a post-programming fuse resistance distribution with less of a low resistance portion is therefore desired to reduce the error rate in electrical fuse programming and thus to increase its reliability of electrical fuse programming.
The critical impact of the doping on the electrical fuse programming has been recognized in the industry and the prior art demonstrates attempts to improve the post-programming resistance of electrical fuses by modifying the structure of electrical fuses through doping. The use of undoped polysilicon or N+ doped polysilicon for all of the cathode, fuselink, and anode have been suggested and tested in the semiconductor industry. The doping of the polysilicon is the same across the cathode, fuselink, and anode in these prior arts.
More recently, U.S. Pat. No. 6,770,948 to Ito et al. discloses electrical fuses with different doping within a “fuse neck” (which approximately corresponds to the fuselink 120 in FIG. 1A and FIG. 1B) for achieving improved post-programming fuse resistance distribution. Among the structures disclosed by Ito et al. include a fuse neck with a P-type doped region and an N-type doped region; P-type doped region, undoped region, and N-type doped region; P-type doped region, region with both P-type and N-type doping, and N-type doped region. According to Ito et al., all interfaces between materials with different doping are confined within the “fuse neck”.
Despite the improvements in the post-programming fuse resistance distribution, reliable programming of electrical fuses still faces challenges as the fuse dimensions shrink and the supply voltage for fuse programming decreases in succeeding semiconductor technology generations. Also, the problem of the statistical occurrence of fuses with low post-programming fuse resistance has been exacerbated by the recent trend in the microelectronics industry that requires reliable programming even under non-ideal programming conditions.
For example, the electrical fuse programming has been performed mostly on a tester before packaging of a chip in an environment where a stable voltage supply is available. The advent of autonomic computing, in which an operating computer can detect defects among its components and repair them during the operation, has created a demand for programming of the electrical fuses under adverse environment, in which the supply voltages may not be as stable as on a tester or the ambient conditions may not be optimal for electrical fuse programming.
Similarly, increased use in hand held devices where the power source is often a battery with a wide range of voltage variations as well as a source of a limited amount of current during the operation has created a demand for electrical fuse programming under adverse environment. Under this type of environment, the distribution of post-programming resistance tends to produce even more low resistance values.
It is therefore highly desirable to improve the design of the electrical fuses to produce a post-programming resistance distribution with less frequency of occurrence of fuses with low post-programming resistance and thus, to provide low programming failure rate. It is also desirable to provide an electrical fuse structure that can produce a distribution of high post-programming resistance even for smaller dimensions.